cytochromes P450 ͉ in situ drug metabolism ͉ in vitro cytotoxicity ͉ on-chip cell encapsulation R ecent advances in genomics and proteomics coupled with sequencing of the human genome have led to a dramatic increase in the number of screenable drug targets (1). Combinatorial (2) and diversity-oriented synthesis (3) programs along with increased access to natural products and their structural scaffolds (4) have provided vast numbers of compounds to screen against these targets. However, there has not been a commensurate increase in the number of approved drugs (5). A major reason for this is the high failure rate of drug candidates because of factors that are not typically considered in the early stages of drug discovery, including poor ADME/tox (absorption, distribution, metabolism, excretion, and toxicology) profiles (6, 7). Thus, pharmaceutical companies are beginning to evaluate toxicity of drug candidates early in the discovery process to reduce the chances of late-stage failure (8).Such early-stage toxicity information requires the development of accurate, reproducible, and predictive in vitro assays. In some cases, validation of in vitro assays has been achieved, such as in percutaneous absorption, skin corrosivity, and phototoxicity (9); however, these tests are limited and are not effective for acute organ-specific toxicity or metabolite toxicity. Moreover, in some European industries (e.g., cosmetics and chemicals), animal testing is being phased out entirely, thereby forcing companies to adopt new in vitro screens that effectively predict human toxicity (10-12). Thus, the need for concordance between in vitro assays and in vivo responses is becoming greater and more pressing, particularly in high throughput that would enable prioritization of compounds for further development involving animal testing of pharmaceutical candidates (13) or direct human testing of cosmetic ingredients.High-throughput screening (HTS) assays for toxicity routinely use 96-or 384-well plates with 2D cell monolayer cultures (14, 15). The multiwell plate format, however, suffers from several limitations, including inefficient removal of reagents from the wells and the difficulty of subsequent washing of cell monolayers (16). These limitations are further compounded when highthroughput screening of cellular targets is coupled with metabolite synthesis, which requires addition of multiple reagents. More recently, to emulate native microenvironments, 3D cell cultures have been used extensively, particularly in tissue engineering applications, e.g., cell-seeded scaffolds (17) and patterned cocultures (18) as well as in directing cell fate and differentiation (19). Although miniaturization of 3D platforms has been performed for high-throughput applications (20, 21), relatively little effort has been directed toward using 3D cell cultures as screening tools for microscale toxicology assays (12,22,23). Herein, we address this technology gap by developing a miniaturized 3D cell-culture array (the Data Analysis Toxicology Assay Chip or D...

The generation of biological diversity by engineering the biosynthetic gene assembly of metabolic pathway enzymes has led to a wide range of "unnatural" variants of natural products. However, current biosynthetic techniques do not allow the rapid manipulation of pathway components and are often fundamentally limited by the compatibility of new pathways, their gene expression, and the resulting biosynthetic products and pathway intermediates with cell growth and function. To overcome these limitations, we have developed an entirely in vitro approach to synthesize analogues of natural products in high throughput. Using several type III polyketide synthases (PKS) together with oxidative post-PKS tailoring enzymes, we performed 192 individual and multienzymatic reactions on a single glass microarray. Subsequent array-based screening with a human tyrosine kinase led to the identification of three compounds that acted as modest inhibitors in the low-micromolar range. This approach, therefore, enables the rapid construction of analogues of natural products as potential pharmaceutical lead compounds.

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An enzyme-containing microfluidic biochip has been developed for the oxidative polymerization of phenols. The biochip consists of a simple T-junction with two feed reservoirs 20 mm apart and a microreaction channel 30 mm long. The channel is 15 microm deep and 200 microm wide at the center, giving a reaction volume of 90 nL. The biochip was fabricated using conventional photolithographic methods on a glass substrate etched using a HF-based solution. Fluid transport was enabled using electroosmotic flow. Soybean peroxidase was used as the phenol oxidizing catalyst, and in the presence of p-cresol and H(2)O(2), essentially complete conversion of the H(2)O(2) (the limiting substrate) occurred in the microchannel at a flow rate of ca. 290 nL/min. Thus, peroxidase was found to be intrinsically active even upon dramatic scale-down as achieved in microfluidic reactors. These results were extended to a series of phenols, thereby demonstrating that the microfluidic peroxidase reactor may have application in high-throughput screening of phenolic polymerization reactions for use in phenolic resin synthesis. Finally, rapid growth of poly(p-cresol) on the walls of the microreaction channel could be performed in the presence of higher H(2)O(2) concentrations. This finding suggests that solution-phase peroxidase catalysis can be used in the controlled deposition of polymers on the walls of microreactors.

We investigated the sensitivity of embryonic murine neural stem cells exposed to 10 pM – 10 μM concentrations of three heavy metals (Cd, Hg, Pb), continuously for 14 days within 3D collagen hydrogels. Critical endpoints for neurogenesis such as survival, differentiation and neurite outgrowth were assessed. Results suggest significant compromise in cell viability within the first four days at concentrations ≥ 10 nM, while lower concentrations induced a more delayed effect. Mercury and lead suppressed neural differentiation at as low as 10 pM concentration within 7 days, while all three metals inhibited neural and glial differentiation by day 14. Neurite outgrowth remained unaffected at lower cadmium or mercury concentrations (≤ 100 pM), but was completely repressed beyond day 1 at higher concentrations. Higher metal concentrations (≥ 100 pM) suppressed NSC differentiation to motor or dopaminergic neurons. Cytokines and chemokines released by NSCs, and the sub-cellular mechanisms by which metals induce damage to NSCs have been quantified and correlated to phenotypic data. The observed degree of toxicity in NSC cultures is in the order: lead > mercury > cadmium. Results point to the use of biomimetic 3D culture models to screen the toxic effects of heavy metals during developmental stages, and investigate their underlying mechanistic pathways.